This article examines the institutional mechanisms affecting the environment and economies of the member countries of international environmental agreements (IEAs), particularly focusing on the legalization and flexibility aspects of IEAs. To identify the factors that influence the consequences of IEAs, we applied the Bayesian probit model to a database including 123 IEAs related to 23 international environmental regimes. The environmental consequences data were taken from the existing database and rescored (Böhmelt and Pilster 2010; Breitmeier et al. 2006), and unintended economic consequences were identified using data from 209 countries. Legally binding IEAs showed a significant improvement of environmental performance, but a significant decrease was related to the presence of inflexible rules. Moreover, decision-making flexibility was positively related to environmental improvement, and negatively related to regime body flexibility. The economic consequences model showed a positive significant impact of the secretariat’s independence on the economies of member countries, while legally binding IEAs showed negative effects. All flexibility elements showed positive impacts on economic consequences. In our research, IEA uncertainty had negative effects on both the environmental and economic aspects; however, we observed positive relationships in the environment and economic analyses when IEAs promoted public goods.